Fix build with glibc 2.27

BUG=806340
TBR=hamelphi@chromium.org

Change-Id: Ib4e5091212d874d9ad88f3e9a1fdfee3ed7e0d5e
Reviewed-on: https://chromium-review.googlesource.com/890059Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Philippe Hamel <hamelphi@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532249}

components/assist_ranker/ranker_example_util.cc

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <math.h>
+
 #include "components/assist_ranker/ranker_example_util.h"
 #include "base/bit_cast.h"
 #include "base/format_macros.h"